1. Field of the Invention
This invention relates to an automatic focusing device that makes measurement of a future object position possible and drives the lens accordingly.
2. Description of Related Art
In the prior art, automatic focusing devices forecasted the future object position by an approximate first order formula (hereafter referred to as a linear function forecast) and calculated the image plane movement velocity from the past to the present defocus amount, the time interval between the two, and the lens movement amount. Known automatic focusing devices forecast the future position of the object by using a quadratic function (hereafter referred to as a quadratic function forecast) from the last three defocus amounts, the time interval of the focus detection, and the lens movement amount during the appropriate time interval.
Even when it is assumed that the object is moving at a uniform velocity, the change in the object position does not occur at a uniform velocity. Because it is assumed that the image plane movement velocity in previous linear function forecasts is uniform, depending upon the disparity from the most recent point of detection time and a future point of time, a large error occurs. For example, in FIG. 1, the object detection position at a previous detection point is P1 at the time of the previous detection T1. The future object position PT at the future point of time TT is forecast from the time of the present detection time T0 of the object detection position P0. As the time difference between the present time T0 and the future time TT increases, the larger the error that will occur, as shown in FIG. 1. When calculating the forecast position of the object at point of time TT by way of a linear function forecast, the position of PT is (erroneously) forecast. Because the actual object is in the PTR position, an error of the magnitude ER occurs.
In addition, because the future position will be forecast by way of an approximate quadratic function from the last three object positions, a large error occurs in the quadratic function forecast due to detection errors and other factors. For example, FIG. 2 shows the quadratic function forecast case where the object position changes from P2, at the first previous detection point of time T2, to P1, at the second previous detection point of time T1, and to P0, at the present detection time T0. In this case, since the detected focal point includes an error, a forecast curve similar to that of FIG. 1 is erroneously forecast and a large error occurs in the future time TT. In other words, when calculating the forecast position of the object at the future time TT by means of a quadratic function forecast, the position PT is erroneously forecast. Because the actual object is in the PTR position, an error of the magnitude ER occurs. Additionally, in the case where the object approaches uniform speed, because the image plane position becomes a hyperbolic function, it cannot be accurately approximated by a quadratic function.